Before going into the field my fellow classmates of 10 and myself received a lesson on stratigraphic columns and how to create one out of an outcrop. Needless to say I was an expert after this one lesson and fairly confident in my capability to rock this. We strut out into the field with our day packs, field notebook and ipads ready to show this massive collection of rocks who’s boss. Well, little did I know that was not me.
It had been only but an hour or two before I began to feel the stress coming on. The beds were becoming harder to define. The sun was in prime position to beat down on hour heads. Some had already taken defeat and were beginning to grow red burnt splotches on their skin. Dripping sweat turned into dried salt stinging our faces. I was slowly but surely beginning to lose confidence in the assignment as I stared at the rocks beds above me. Then the TA approached me asking to take a look at my notes. He then explained to me that my drawings were incorrect. It was then I could feel the real frustration brewing. I wanted to give up but instead I took a breather and got back to work.
To avoid this kind of frustration for later generations I have outlined a wiki-how on “Keeping your cool while designing stratigraphic columns”.
Before you go out into the field you will need to collect these necessary items.
A grid paper notebook
a grain size card
a meter ruler
a hand lens.
Things highly recommended would be:
a full bottle of sunscreen (unless that is you want to burn)
full water bottle
Draw out the stratigraphic columns. It is recommended you draw a couple of these before you get out into the field, this will help reduce stress levels later. The graph consists of 5 columns. The first one marking which bed you’re focusing on; the second is the height of the bed; the third is going to be a sketch of the bed; the fourth column is dedicated to detailed descriptions in macro, meso and micro scale and the fifth is your interpretation how the rocks came to settle that way. The vertical axis will be the height in meters of each bed and the horizontal axis will be the length in millimeters of the actual clasts themselves.
This is where the real fun begins. Settle your things on any piece of land. Try not to spend time trying to find a less ashy place, there is no such thing, trust me. Now that you’re in front of your outcrop step back about 30 feet and begin to look at it from a macro scale.
Take down description of color, for example, is it tan, maybe greyish? Perhaps there is staining in the pumice due to oxidation. Then take down the different contact or lenses you find. Contacts separate the different beds from each other and each bed represents a different volcanic event. A sharp contact is one of the easiest to see. It is a clear change in the event that caused that layer. And erosive contact happens when a stream starts to cut down on rocks and can cause a truncation or dip in the contact. A gradational contact comes from slow changes of processes of the eruption switching back at forth.
Good job you’ve made it to the fourth step and you aren’t confused yet! Or maybe you are, but that’s all part of the fun! This is just the beginning so get strapped in. What we are looking at now is the mesoscale. Now we must examine what types of rock compose the bed we are looking at.
Get close enough to touch the bed. Pull out the meter ruler and take down a measure of the height of the bed you are examining. Then determine what kind of rocks make up the bed. Then take out the handy dandy grain size card and begin to find an average size for each type of rock. From here we are going to determine the amount of lithic fragments. This will help us to tell whether the bed is well or poorly sorted. This can be found on your grain size chart as well. If it is well sorted everything will be close to the same size, if poorly the grain sizes will be varying highly along with a large amount of lithic chunks.
From here we can also see how the clasts are supported. If we describe them as clast supported then all of the rocks of the same clasts are touching. If they aren’t, then they will be matrix supported. You will then determine the type of grade the bed has. Normal grading is when the larger rocks are at the bottom and the smaller are on top. Reverse grading is when the smaller rocks are at the bottom and the larger are toward the top. This can happen through water saturation or how efficiently the gas is released during an eruption.
Don’t worry the stuff dripping down your back is not collective ocean spray but the water you drank evaporating out of your skin. But its okay, take a sip of water and reapply sunscreen to your crispy skin and let’s continue.
This next step consists of looking at the clasts through the microscale. Take out your hand lens and begin looking at sample rocks in order to describe them in finer detail. Are there any vesicles in the sample? What about phenocrysts? Both of these descriptions tell us in what conditions these rocks were formed. For example, if there are many vesicles present it means that the magma was gas rich and pressure was rapidly decreased when the volcano erupted. If there are phenocryts it means that the rock cooled slower allowing it time to form crystals.
Now that you have analyzed the bed you need to begin to draw it. First determine a set key to let someone know the significance of a squiggle (pumice) or a group of dots (ash). In your drawing you will mark the height of the bed and the variation in rock length set by the vertical and horizontal axis respectivley. Remember in order to show the size of the rocks you must draw the bed to size with the millimeters on the bottom. If you find that the bed also had lithic fragments make sure to add those in too.
While drawing the bed you want to make sure it accurately represents exactly what you are seeing. So if there are altering layers you would want to put altering layers of dots and squiggles in the drawing.
This part of the stratigraphic column might take a while but don’t worry no one gets it right the first time.
You have made it to the last step. Wipe off that sweat and wear your gnarly tan lines with pride. In this step you will write an interpretation of the bed and how you think it came to be. For example, “The bed is reverse graded because the pumice fell out of suspension from water, causing small pumice to saturate faster and bigger pumice slower”. Everyone’s interpretation will be different since this part is more opinion based.
Congratulations, you have now successfully created a stratigraphic column! To make another just repeat steps 1 through and remember to take deep breaths in between each step. Good luck and happy stratigraphing!